Basic electrostatics (geometry help)

[geft]

Here's the problem: http://i.imgur.com/dS7ZH.jpg

I don't understand why the cosine has to be there. My geometry is a bit rusty, but wouldn't it be enough without the cosine part?

 

[NascentOxygen]

E is the vector sum of the 4 E's. The vertical component of each E_o at the point of interest is canceled out by the vertical component there due to an identical charge at one other vertex, leaving only horizontal components to be added. The cos gives you that horizontal component of each E.

 

[geft]

Ah, you're right. Thanks!